1. Field of the Invention
The present invention relates to an electronic flash device in which the amount of light emitted therefrom can be controlled, and more particularly relates to such an electronic flash device which stops its light emission in response to a signal supplied from a camera when the integration of the light reflected from a film being exposed reaches a given value.
2. Description of the Prior Art
To make more convenient flash photography with an electronic flash device (referred to as strobe hereinafter) that stops its light emission in response to a stop signal supplied from a camera, a flash photography system has been proposed wherein, when a desired amount of light has been reached to generate the stop signal and stop the light emission of the flash tube before the flash tube has fully or entirely emitted its light, a specific indicating element arranged to be seen in the camera viewfinder field is energized intermittently or continuously to indicate to the user that the flash photography at that time has been made properly within the capacity of the strobe to provide a desired amount of exposure.
The present invention improves the strobe used in such a flash photography system. As is well known in the art, the shutter speed must be set to a flash synchronizable value e.g. 1/60 second or less for flash photography with a focal plane shutter camera. The flash tube of a strobe emits light for about 1 millisecond at the most which is considerably shorter than the flash synchronizable shutter speed. Thus, in the photograph with the flash synchronizable shutter speed, the film is exposed to the light of a scene under the illumination of ambient light for the period the flash tube does not emit light. Accordingly, it sometimes possibly occurs that a proper exposure is obtained after the flash tube has emitted its light fully, i.e. after the main capacitor has been substantially discharged, with the scene light due to the ambient light illumination compensating for the shortage of the exposure by the flash light.